In gas turbine engines it is now common practise to cool the surface of airfoils, such as the rotor blades, by passing high pressure air or other gas from the turbine compressor through channels in the airfoil and out through small holes in the wall of the airfoil, thereby providing a layer of cooling gas over the airfoil surface. It is also known that the effectiveness of the layer or film of air or other gas in cooling the surface is increased by enlarging the opening of the holes at the surface of the airfoil, such holes with enlarged openings being known as diffusion film holes.
The background of this field of gas cooling of airfoils is described further in U.S. Pat. No. 4,197,443, of J. E. Sidenstick, and in U.S. Pat. No. 3,795,604, of L. J. McKinney et al.